1. Field of the Invention
The present invention relates to an electrode for plasma generation having the capability of stably providing uniform plasma without unusual discharge and an increased durability of electrode, plasma treatment apparatus using the electrode, and a plasma treatment with the apparatus, which is particularly suitable for surface-cleaning glass substrates for liquid crystal display (LCD), electronic parts and so on.
2. Disclosure of the Prior Art
In the past, plasma surface treatments are performed to objects to be treated by use of plasma generated by applying an AC voltage between electrodes in a plasma-generation gas atmosphere such as argon and helium to modify or improve surface conditions of the objects. In the plasma surface treatments, it is important to prevent contamination of the objects with impurities such as spattered electrode materials, and increase the life of the electrodes exposed to plasma.
For example, Japanese Patent Early Publication [KOKAI] No. 6-96718 discloses an electrode for glow-discharge plasma generated under atmospheric pressure, which is characterized in that a plurality of metal pipes each having a ceramic-sprayed coating on its outer surface are arranged parallel with each other at a required interval between adjacent metal pipes. The ceramic-sprayed coating provides an increased electrode life and reduced contamination of the objects. However, in general, there is a problem that pinholes easily occur in the ceramic-sprayed coating.
The pinholes become a cause of poor withstand voltage of the ceramic-sprayed coating. That is, an unusual discharge such as spark discharge easily occurs in the vicinity of the pinholes. In such a case, a serious breakage of the ceramic-sprayed coating is often caused. In addition, since uniform discharge can not be stably obtained, the efficiency of plasma treatment lowers. Moreover, when some of the objects are heated at a high temperature by the unusual discharge, they may become defective pieces. Thus, the ceramic-sprayed coating still has the above problems to be improved.
On the other hand, Japanese Patent Early Publication [KOKAI] No. 11-191500 discloses an electrode for glow-discharge plasma, and a surface treatment with the glow-discharge plasma. The electrode comprises an electrode substrate and an alumina-based sintered ceramic having a purity greater than 99.6% coated on the electrode substrate. This electrode is suitable to generate stable glow discharge under atmospheric pressure. However, since the sintered ceramic is produced by means of powder metallurgy, and then is bonded to the electrode substrate by use of an adhesive, there is a problem that the adhesion between the sintered ceramic and the electrode substrate is poor.
From the viewpoints of plasma stability and durability of electrode, the electrodes for plasma generation of the past still have plenty of room for improvement.
Therefore, a primary object of the present invention is to provide an electrode for plasma generation having the capability of stably providing uniform plasma without unusual discharge, and an increased durability of electrode. That is, the electrode of the present invention comprises an electrode substrate and a protection layer formed by heat-fusion coating a glass-based material on at least a surface exposed to plasma of the electrode substrate.
It is preferred that a withstand voltage of the protection layer is within a range of 1 to 50 kV. It is also preferred that the protection layer contains silica, alumina and titania as main ingredients. Moreover, it is preferred that the protection layer contains 30 wt % or less of at least one of alkali and alkaline earth metals.
It is also preferred that the electrode substrate is of a tubular structure, and the protection layer is formed on an outer surface of the tubular structure. In this case, it is preferred that the electrode substrate has a corrosion resistance layer on an inner surface of the tubular structure. In particular, it is preferred that the electrode substrate is a seamless pipe of an electrode material, and the protection layer is formed on an outer surface of the seamless pipe, or the electrode substrate is of a tubular structure, which is formed by preparing a rectangular plate of an electrode material, and densely welding a side of the rectangular plate to the opposite side by means of voidless welding, while preventing the occurrence of voids at a welded portion.
Another object of the present invention is to provide a plasma treatment apparatus for treating an object placed in a discharge space with a dielectric barrier discharge plasma. The apparatus comprises at least one pair of electrodes for plasma generation, a treatment chamber for accommodating the electrodes therein, gas supply unit for supplying a plasma-generation gas to the discharge space defined between the electrodes, and a power supply for applying a voltage between the electrodes to generate the dielectric barrier discharge plasma of the plasma-generation gas in the discharge space. The apparatus of the present invention is characterized in that at least one of the electrodes comprises an electrode substrate and a protection layer formed by heat-fusion coating a glass-based material on at least a surface exposed to plasma of the electrode substrate.
It is preferred that the electrode is provided with the electrode substrate having a tubular structure, the protection layer on an outer surface of the tubular structure, and a corrosion resistance layer on an inner surface thereof. In addition, it is also preferred that a distance between the electrodes is within a range of 1 to 20 mm.
A further object of the present invention is to provide a plasma treatment with the plasma treatment apparatus described above. The plasma treatment is characterized in that the object is treated by the dielectric barrier discharge plasma of rare gas or a mixture of the rare gas and a reactive gas generated in the vicinity of atmospheric pressure.
These and still other objects and advantages will become apparent from the following detail description of the invention and Examples of the invention.